This invention relates generally to feedback systems in which cancellation techniques are applied to the feedback path. One useful application of these techniques permits suppression of undesired distortion generated by devices such as amplifiers.
In recent years, techniques have been developed which improve linearity of amplifier designs using feed-forward cancellation of undesired amplification artifacts (distortion). Numerous examples of feed-forward distortion cancellation are taught in the existing body of patents. While generally providing good distortion suppression, feed-forward techniques have several drawbacks. They tend to be complex; they require an error amplifier; and they usually require addition of a delay element in the output of the main amplifier. Because the error amplifier must be highly linear, it will consume a significant amount of power thus lowering the overall efficiency of the amplifier. The delay element is used to broadband the amplifier. This element must be low loss or overall amplifier efficiency will suffer. Low loss delay elements are generally physically large and thus tend to set a lower bound on amplifier size.
Proper application of feedback cancellation techniques can eliminate the aforementioned drawbacks associated with feed-forward cancellation. Feedback cancellation is described in U.S. Pat. No. 5,396,189 to Hays (1995) which discloses a method and apparatus for adaptively removing selected signals from the feedback path leaving only distortion to be negatively fed back. By this means, both in-band and out-of-band distortion such as intermodulation products and noise can be suppressed without reduction in effective amplifier gain.
A prime object and advantage of this invention is to provide a flexible alternative to the vector detector/controller combination taught in U.S. Pat. No. 5,386,189 which does not rely upon the comparison of a reference signal with a feedback signal. While application of feedback cancellation to the suppression of amplifier distortion is emphasized herein, the method and apparatus are not limited to this particular application and can be applied to closed-loop control systems in general.
Another object and advantage is to provide a means and method to improve cancellation bandwidth of the feedback signal via incorporation of a network into the reference signal path.
The term xe2x80x9cdistortionxe2x80x9d as used herein refers to any signals present at the output of a device which were not present at its input. Distortion may include such unwanted signals as noise, intermodulation products, harmonics, and so on. The term xe2x80x9cdesired signalxe2x80x9d as used herein refers to the signal or signals which the device is intended to pass and, depending on context, will refer to either signals at the input or output of the device. And the term xe2x80x9cin-bandxe2x80x9d as used herein refers to a frequency band sufficiently wide to pass the desired signal.